Despite abundant research on the distribution of word-initial glottal stops, it is still unclear which factors matter most in predicting where glottal stops occur and why. In this study, logistic mixed-effects regression modeling is used to predict the occurrence of word-initial full glottal stops in an English corpus. The results indicate that prominence and phrasing are overwhelmingly the most important factors in predicting full glottal stop occurrence. Moreover, prominent word-initial vowels that are not preceded by a glottal stop show acoustic correlates of glottal constriction, whereas non-prominent phrase-initial vowels do not. Rather, phrase-initial voicing (even for sonorants) is less regular, but in a manner inconsistent with glottal constriction. These findings are subsequently con_rmed using articulatory measures from electroglottography, and extended to Spanish. Based on the results, a prominence-driven theory of word-initial glottalization is proposed and motivated, with higher phrasal domains responsible for the strength of the glottal stop gesture.